Summary

Forecasting crop nitrogen (N) demand is important for maximising productivity and minimising losses to the environment and includes taking into account residual effects. The residual N effect was estimated in a dairy crop rotation (spring barley undersown with grass-clover, 1st and 2nd year ley, spring barley/pea undersown with ryegrass, oats undersown with ryegrass and fodder beets) with different management (grazed/cut) and manure type (slurry/deep litter) by anaerobic incubation and plant N-uptake in a pot experiment and in the field. For comparison a 10-year-old grass-clover was included. Type of animal manure did not affect the residual N effect. Crop rotations with grazed grassland had a residual effect that on average was 13% higher than the same rotation without grazing. Ploughing of grasslands clearly increased residual N effects for several years, but grassland age at ploughing was of little importance. Thus, the residual effect of 10-year-old grass-clover ley only marginally exceeded that of undersown grass-clover despite considerable difference in estimated accumulated N-surplus. The results indicate that organic N is easier to mineralise the more recently it has been formed. Good correlations existed between soil inorganic N in the spring, N released during anaerobic incubation and plant-available N but chemical measures may be difficult to implement in practical farming due to difficulty in representative sampling in systems characterised by huge spatial variability.